1. Field of the Invention
The invention relates to a device for digital speed control of an asynchronous motor via a static converter.
2. Brief Description of the Background of the Invention Including Prior Art
In many cases, a very accurate automatic control of drive motors for transport and heating galettes of textile machines is required to assure a uniform yarn quality. In addition, numerous other application areas of electric motors require the same features.
In such cases, direct-current motors as well as synchronous motors are employed. However, both are costly and the required control devices are also expensive. Asynchronous motors, which are fed via a static power converter, are much more economical.
A rotation speed control circuit as set forth above is known from the U.S. Pat. No. 3,887,853. The first input of a first summator is fed from the output of the digital to analog converter and the second input is fed from the output of a frequency/voltage converter receiving rotation-speed set point pulses. A second summator follows to this first summator, which receives its second input signal from a special attenuation circuit. The attenuation circuit receives rotation-speed actual-value pulses from the rotation-speed measurement device connected to the motor and converts these into a voltage proportional to the rotation-speed actual value. This voltage is fed, together with the output signal of the second summator, to the two inputs of a third summator. A delay circuit and a filter network follow the third summator. The output of the filter is connected to a second input of the second summator. In this manner, an internal integrating negative or inverse feedback circuit is formed, which opposes the phase swinging or pendulum oscillations and the resonance phenomena of the rotation-speed control circuit and of the motor. The output signal of the second summator controls the power inverter.